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The use of hazardous solvents such as perchlorethylene (xe2x80x9cPERCxe2x80x9d), a chemical suspected by the Environmental Protection Agency (xe2x80x9cEPAxe2x80x9d) to be a carcinogen, in commercial dry cleaning systems has come under increased scrutiny in recent times. The environmental regulations and liability considerations of current solvents has generated a search for an alternative process that can compete from both an economic and performance standpoint, while remaining environmentally friendly. Alternative solvents have been proposed, most notably liquid carbon dioxide (LCO2), which is available as a by-product from a variety of industrial processes, including fertilizer manufacturing.
To date, systems employing LCO2 have either used a single LCO2 supply tank in conjunction with a cleaning vessel, or twin LCO2 supply tanks in mutual communication with a cleaning vessel. Most such systems have employed a heavy-duty, positive-displacement piston pump to provide a substantially continuous flow of LCO2 through the respective system during substrate agitation.
In order to address various deficiencies associated with the use of such pumps, compressors have been proposed to circulate LCO2 between a storage tank or tanks and a cleaning vessel by means of pressure differentials, obviating the need for a pump. In a single-storage tank embodiment, the compressor is employed to convey solvent to the cleaning vessel prior to agitation, then back into the storage tank after agitation; agitation itself is achieved through the use of some mechanical means, including a rotating basket or paddles, in a single-storage tank embodiment.
In a two-storage tank embodiment, a positive pressure differential enables the flow of LCO2 from one storage tank to the cleaning vessel and thence to the second storage tank. The direction of solvent flow is then reversed in order to maintain the flow of solvent through the cleaning vessel. Here, the introduction of at least a portion of the liquid solvent through nozzles in the cleaning vessel results in jet agitation of the substrates. The magnitude of the pressure differential between one storage tank and the other may be controlled by varying the speed of the compressor motor or by using a throttle valve. The compressor may also be used to draw gaseous LCO2 from one storage tank into the other storage tank in order to create the pressure differential.
In the prior art, it is necessary to heat gaseous CO2 as it is being conveyed into the cleaning vessel during pressure equalization; as the pressurization of the gaseous CO2 decreases in a first storage tank, the temperature in the first storage tank drops. This effect may be exacerbated if the cleaning vessel has been pumped down to remove water vapor prior to pressure equalization. Thus, the remaining LCO2 in the first storage tank is at a temperature which is below optimal for dry cleaning purposes, requiring it to be heated prior to being transferred into the cleaning vessel for substrate agitation.
Heating the LCO2 for this purpose could be done through the use of a heat exchanger in the fill line. Alternatively, one could start with a storage tank some 20 degrees C. above the target range, but this would result in significantly higher pressures, and would require a higher pressure-rated storage tank, which is of course more expensive and potentially bulkier.
At the end of the cleaning cycle, it is necessary to evacuate gaseous carbon dioxide from the cleaning vessel into one of the storage tanks. To convert carbon dioxide vapor in the cleaning vessel into a liquid for storage following a cleaning cycle, the vapor must be cooled to avoid an excessive increase in pressure.
Thus, prior art two-tank systems which exchange LCO2 through a cleaning vessel require the liquid cleaning medium to be heated prior to introduction into the cleaning vessel and the gaseous carbon dioxide vapor to be cooled as it is returned to one or both of the storage tanks.
Cooling the vapor to a degree necessary to liquefy it requires a very large refrigeration system. Absent such a system, an overpressure condition might result as the vapor is pumped back into the storage tank. Plural heat exchangers with hot water and cold water reservoirs and pumps may suffice for this purpose, but are expensive and result in added system complexity.
A dry cleaning system is disclosed which in a preferred embodiment utilizes liquid carbon dioxide as the cleaning medium. Two storage tanks are employed, one of which is relatively xe2x80x9ccoldxe2x80x9d and the other being relatively xe2x80x9chot.xe2x80x9d These tanks are alternatively referred to herein as the xe2x80x9cthermo tankxe2x80x9d and the xe2x80x9csolvent tank,xe2x80x9d respectively. Substrate washing is performed in a cleaning vessel, which for liquid carbon dioxide is maintained at 20-24 degrees C.
After loading the substrates to be washed, such as clothing, into the cleaning vessel, the pressure in the thermo tank and in the cleaning vessel is equalized by placing the cleaning vessel and thermo tank in vapor communication. The temperature of the residual solvent, which remains in the thermo tank throughout the cleaning process, is allowed to drop as the pressure decreases. A compressor is used to force additional gaseous solvent into the cleaning vessel, raising the pressure therein to a point closer to the internal pressure of the solvent tank. Then, the solvent tank and the cleaning vessel are placed in fluid communication so that the cleaning vessel is filled with LCO2 through operation of the compressor. It is preferred to pressurize the cleaning vessel by connecting the thermo tank to the cleaning vessel prior to filling the cleaning vessel with LCO2, otherwise ice or xe2x80x9csnowxe2x80x9d would form in the cleaning vessel, which may block the lines and valves to the cleaning vessel.
Once the thermo tank is placed in vapor communication with the cleaning vessel, the temperature of the liquid carbon dioxide in the thermo tank drops as some of it vaporizes during pressure equalization. This drop can be 20 degrees C. lower than the starting temperature. Then, as further gaseous CO2 is compressed out of the thermo tank and into the cleaning vessel, more liquid CO2 evaporates, resulting in a further temperature drop on the order of 40 degrees C. Thus, the total drop in temperature in the thermo tank is close to 60 degrees C. This effect may be increased in one embodiment where the cleaning vessel has been pumped down to xe2x88x9214 psi initially to remove water vapor which would otherwise have a deleterious effect on substrate cleaning. In other cases, however, the amount of water vapor in the cleaning vessel initially may be so small as to not require initial evacuation.
At the completion of substrate agitation, LCO2 is transferred back into the solvent tank, following which gaseous CO2 is extracted and condensed into the thermo tank. This process reduces the temperature of the cleaning vessel and substrates to the point where damage can occur to the cleaning vessel contents; some plastic and vinyl materials crack at sub-freezing temperatures. Clothing is also more prone to wrinkle at lower temperatures.
Conversely, at the end of the cleaning cycle, the gaseous CO2 which is removed from the cleaning vessel becomes hotter as a result of compression. In order to employ this latent heat energy, the return line from the cleaning vessel to the thermo tank is routed back into the cleaning vessel where it forms a heat exchange coil below a rotary basket used for substrate agitation. In order to raise the temperature of the residual LCO2 in the thermo tank and complete the condensation of the hot, compressed, gaseous CO2 extracted from the cleaning vessel, the gaseous CO2 is introduced back into the thermo tank through a sparging tube, such that small gas bubbles of heated CO2 efficiently transfer heat to the liquid-phase CO2.